1. Field of the Invention
The present invention relates generally to electrophotographic and electrostatic recording elements and processes for making the same; and more particularly to an improved technique for making an electrical ground connection to the intermediate electrically-conductive layer of such elements.
2. Description of the Prior Art
As used in the present specification, the term "electrostatographic" is intended to mean and cover both electrographic and electrophotographic members.
Both types of recording elements are employed in reproduction processes. Essentially, a latent image is formed in these elements by providing an imagewise surface charge on an insulating surface of the element, and thereafter developing the charged latent image by means of an electrically-attractable material, such as a "toner" (i.e., directionally charged colloid carbon particles suspended in an insulating liquid or on a dry carrier).
Specifically, an electrophotographic element normally comprises at least a substrate having coated thereon a layer containing a suitable photoconductor. The support layer is made conductive either by the inclusion therein of electrically-conductive materials or by coating its surface (the surface designed to receive the photoconductive layer) with an electrically-conductive material. Images are usually formed on the photoconductive layer by first applying a uniform electrostatic charge to the photoconductive layer by any suitable method, thereafter imaging the charged material by exposing it to light through a transparent master or by reflection from an opaque master which is being reproduced, thereby causing the photoconductive layer to become conductive, resulting in the dissipation of the charge in those areas of the layer exposed to light. In a subsequent step, the charge pattern or latent image on the image layer is rendered visible by the application thereto of a colored or black electroscopic toner.
An electrographic recording element is similar in construction to the electrophotographic element, with the photoconductive layer being replaced with a high dielectric layer or a "charge retentive layer", that is, a material having a volume resistivity of not less than about 10.sup.12 ohm-centimeters. During this "printing" process the electrical charge is applied to the image areas by a stylus and developed in the same manner as the electrophotographic element.
With either type of element, it is often desirable during one or more of the processing steps (e.g. during charging or during toning) to establish a ground connection (to a reference potential) to the conductive layer of the recording element in order to create a highly conducting reference plane which is held at or near ground potential. For example, during the electrostatic charging of the photoconductive layer or during the "printing" of the dielectric layer, the potential of the conducting layer has a tendency to build-up with respect to ground if it is not grounded. If this should occur, then there is a less than desirable difference in potential between the areas struck by light and those not struck in an electrophotographic element, and between the areas retaining or not retaining "print" charges in an electrographic element, resulting in a latent image which is difficult to develop in the subsequent toning step,thereby resulting in copies which exhibit poor image quality, high background discoloration and/or poor contrast.
The prior art is replete with suggestions as to how to assure proper electrical grounding of the conductive layer in such elements. The classical approach is to provide an electrically-conductive substrate which is easy and conventional if the substrate is a permeable or non-homogeneous substrate such as paper, since the substrate can be filled during manufacture with an electrically-conductive pigment such as carbon black, or saturated with a solution of an ionic, hygroscopic substance, such as salt of a polyelectrolyte. However, there are disadvantages in such a technique. For example, the ionic hygroscopic impregnant is not generally effective in low porosity papers and the conductivity it imparts is not stable but varies by several orders of magnitude with the relative humidity of the environment. The use of carbon black as a filler insures the stability of the conductivity, but may be unacceptable for aesthetic reasons especially if a white substrate is desired. Moreover, it tends to weaken the substrate physically. Finally, it must be formulated into the substrate at the time of manufacture and thus, cannot be applied to separately-made substrates. In addition, the primary disadvantage of this technique is that it cannot be applied to homogeneous, impermeable substrates, such as extruded or cast films made from, for example, synthetic resins such as polyesters.
A further approach has been to provide a conductive intermediate layer between a non-conducting substrate and the photoconductive or dielectric layer. It has been suggested to contact this conductive intermediate layer by removing the dielectric or photoconductive layer from a limited area thereby exposing the intermediate conductive layer so that an electrode can be physically contacted with the conductive layer. Other approaches have been to cut, punch, scratch or otherwise disrupt the physical integrity of the dielectric or photoconductive layer with an electrode which is forced into contact with the intermediate conductive layer.
A further alternative of the prior art has been to provide a conductive pathway established from the conductive layer to the outer surface of the member where an electrode can establish contact with it. Since it is a part of the recording member, it must be inexpensive to apply, inconspicuous, flexible (in flexible recording elements) and of course both effective and reliable.
An example of the prior art's solutions to this problem is shown in U.S. Pat. No. 3,118,789, Wiswell et al. The patent discloses an electrophotographic recording member comprising a paper substrate, a conductive interlayer and a photoconductive layer overlying the conductive interlayer with, optionally, a conductive layer coating the back side of the paper substrate. The paper substrate is perforated with fine holes so that the conductive lacquer of which the conductive interlayer is composed will penetrate through the paper during the coating operation and thus enable the establishment of an electrical connection therethrough to the conductive interlayer. A disadvantage with this type of electrical connection is that it is not useful with impermeable substrates such as polyester films. If the substrate is a film, and transparency and use as a photographic member is required, the methods suggested in the Wiswell et al patent cannot be employed to establish contact with the conductive interlayer because of the damage such a technique will cause to the film's optical characteristics.
Another solution to the problem is disclosed in U.S. Pat. No. 3,639,121, York. The patentee discloses electrophotographic elements comprising a support, an intermediate conducting layer and an uppermost photoconductive layer wherein electrical contact to the conducting layer is accomplished by coating the edge of the laminate with a conducting lacquer, which coating can be connected to ground to thereby electrically ground the intermediate conducting layer.
U.S. Pat. No. 3,533,692, Blanchette et al, discloses a photoconductive medium wherein the photoconductive layer is removed from an area to expose the conductive interlayer, with which contact is then made with an electrode, or in which a conductive metal strap or rivet is used to establish an electrical connection between the interlayer and a conductive area coated on the back of the laminate.
U.S. Pat. No. 3,552,957, Hodges, discloses the use of mechanical devices such as straps, clamps and rivets applied to an exposed area of the conductive interlayer of an electrophotographic recording member to establish electrical connection with the interlayer.
Similarly, U.S. Pat. No. 3,574,615, Morse, discloses a connection scheme as in Hodges except that an electrically-conductive elastomeric pad is inserted between the clamp-electrode and the exposed surface of the conductive interlayer.
Further, U.S. Pat. No. 3,543,023, Milin et al, eliminates the necessity for stripping the dielectric or photoconductive layer away to expose the conductive interlayer, and discloses instead the use of a corona discharge at the edge of the member to establish the necessary electrical connection.
A disclosure related to the above discussed York patent appears in U.S. Pat. No. 3,684,503, Humphriss et al. Humphriss et al achieves the necessary electrical connection by providing, in a non-recording section of the element, a solid dispersion of a particulate electrically conducting material which extends from an external surface of the element through a portion of at least one of the layers overlying the conductive layer to electrically contact the conducting layer. Alternatively, the dispersion can extend into the element from an edge thereof.
Although the prior art recording members generally function as intended, there has been a need for a technique of providing the necessary electrical ground connection to the conductive interlayer in an electrostatographic recording member which is simple to fabricate and yet is effective in establishing good electrical connection with the conductive interlayer.